At first, before PostCSS became
popular, cssnext was designed to be a complete tool. Thus, cssnext included some options that don’t really belong in a PostCSS
plugin focused on the future of CSS
(e.g., import, url, compress, plugins…) .

These days, most people use PostCSS
directly (so they can easily adjust and choose their CSS transformations).
So we decided to make the integration of cssnext simpler by providing a
simple (real) plugin.

Also, having to maintain several cssnext and PostCSS runners that do almost
the same thing is not optimal.